1. Field of the Invention
This invention relates to a braking system for a vehicle in which both regenerative and friction braking subsystems are used. A system according to this invention will have the capability of adapting to varying road surface conditions because such a system will be able to calculate the maximum torque which the braking system may generate at any particular roadwheel or set of roadwheels, using either the regenerative braking system alone, or both of the systems, without causing skidding or slipping of the roadwheels.
2. Disclosure Information
Anti-lock brake systems, which are sometimes termed "anti-skid", have come into prominence in the automotive world. Designers have devised a number of control algorithms and strategies for preventing braked wheels from slipping or skidding in the event that the available brake torque exceeds the torque capacity of a roadwheel. Various types of anti-lock braking systems (hereinafter "ABS") have been proposed as a solution for specific problems in applying braking systems. U.S. Pat. No. 4,735,279, for example, deals with application of ABS to a 4-wheel drive vehicle. U.S. Pat. Nos. 4,491,919 and 4,652,060 deal with problems associated with diagonally split brake systems in which, for example, the left front and right rear roadwheels are connected in a common braking system. U.S. Pat. Nos. 4,733,921 and 4,740,040 disclose ABS systems useful for controlling vehicles on so-called split coefficient of friction surfaces which may, for example, be encountered where one set of roadwheels e.g., the right wheels of the vehicle, may be on ice, whereas the left set of roadwheels is on dry pavement.
As with most technologies, improvements upon basic concepts abound. Accordingly, U.S. Pat. No. 3,832,009 discloses an ABS including a single evaluation channel for processing wheel speed information from multiple roadwheels. U.S. Pat. No. 4,374,421 is concerned with yet another strategy for controlling the left versus the right wheel on a single axle.
None of the foregoing systems deals with the control issues related to a vehicle having regenerative braking capability in which an electrodrive traction motor is operated both as a motor to thereby drive the vehicle by absorbing energy from a battery, and as a generator to thereby brake the vehicle and to simultaneously convert kinetic energy to electrical energy for a purpose of recharging the traction battery.
U.S. Pat. Nos. 3,621,929 and 4,671,577 deal with the combination of friction and regenerative braking in a vehicle. Neither patent, however, teaches or suggests a system for maximizing regenerative braking while at the same time preventing skidding by wheels braked either regeneratively or by means of friction mechanisms. More specifically, the '929 patent is devoid of any teaching or suggestion that torque created by the braking systems could or should be limited so as to avoid skidding or wheel locking, whereas the '577 patent, although teaching the coordinated use of friction and regenerative braking in the event that wheels braked regeneratively begin slipping, is devoid of any teaching that braking effort produced by the friction and regenerative means could or should be proportioned according to the coefficients of friction present at the various driven and undriven roadwheels. The '577 patent thus provides a braking effort which is developed in the same proportion by friction and regenerative means for all braking rates.
It is an object of the present invention to provide a regenerative friction braking system which is adaptive inasmuch as the present system allows proportioning of brake torques according to the individual coefficients of friction pertaining to each of the brake controlled roadwheels.
It is an advantage of the present invention that a system according to this invention will allow maximum use of regenerative braking without causing locking or skidding of either powered or unpowered roadwheels.
It is a feature of the present invention that a system according to this invention will periodically recalculate the coefficient of friction existing between the roadwheels and road surface so as to generate the capacity for maximum safe brake operation.
Other objects, features and advantages of the present invention will become apparent to the reader of this specification.